I can here you say, “Everybody knows this already.” However for those of you who don’t (non developers, artists maybe) there are the sizes needed to create new screen shots for iTunesConnect app metadata and marketing. Apple hasn’t produced an iOS 8/iPhone 6 transition guide and the Mobile HIG hasn’t been updated, yet.
iPhone 6 – 755 x 1334
iPhone 6 Plus – 1242 x 2208
The iPhone 6 Plus also has landscape mode so you may want to create screen shots for that mode as well.